The present invention relates generally to sewing machines equipped with a fabric holder (i.e., fabric-holding member) vertically movable together with a needle bar, and more particularly to an improved sewing machine which allows a sewing workpiece to be reliably held down by a fabric holder when the fabric holder is located at its lower dead point and also allows the sewing workpiece to be held down by the fabric holder with a constant holding force.
Heretofore, there have been known sewing machines equipped with a fabric holder supported on a needle bar in such a manner that the fabric holder is vertically movable relative to the needle bar that is vertically driven in a reciprocative fashion. The fabric holder is normally biased or urged downward by a coil spring provided on the needle bar so that its abutment portion abuts against a sewing-needle mounting member (so-called “needle clamp”) that is provided on the needle bar and also functions as a stopper. The fabric holder is vertically moved together with the needle bar while kept abutting against the sewing-needle mounting member (needle clamp), but, when the fabric holder has reached its lower dead point to hold down a fabric or sewing workpiece, only the needle bar moves with the fabric holder staying at the lower dead point. One example of such a sewing machine is known from Japanese Patent Application Laid-open Publication No. HEI-5-245278 or No. HEI-9-84981. The No. HEI-5-245278 publication discloses a sewing machine where a needle-bar driving shaft (also known as “main machine shaft”) is oriented to extend in a left-right horizontal direction of the sewing machine, while the No. HEI-9-84981 publication discloses a sewing machine where a needle-bar driving shaft is oriented to extend in a front-rear horizontal direction of the machine (so-called “arm-type sewing machine”).
In the sewing machine disclosed in each of the No. HEI-5-245278 and No. HEI-9-84981 publications, a supporting member for supporting, from below, a predetermined engagement portion of the fabric holder during the downward or descending movement of the fabric holder is provided to lessen a noise sound (collision sound) generated during integral vertical movement of the fabric holder and needle bar; the supporting member is vertically driven in synchronism with the vertical movement of the needle bar. More specifically, the supporting member is reciprocatively driven, in synchronism with the vertical movement of the needle bar, between a lower limit position where the supporting member supports the engagement portion of the descending fabric holder to define the lower dead point of the fabric holder and an upper limit position spaced upward a predetermined distance from the lower limit position. In the descending stroke of the needle bar, the supporting member is driven, downward from the upper limit position to the lower limit position, in an appropriately-controlled manner so as to support the engagement portion of the fabric holder at the lower limit position with a minimized relative-moving-speed difference between the fabric holder and the supporting member; in this way, it is possible to effectively avoid a sound of collision between the engagement portion of the fabric holder and the supporting member when the fabric holder has been stopped at the lower dead point. In the ascending stroke of the needle bar, on the other hand, the supporting member is driven to move upward, during a period before the needle clamp of the needle bar comes immediately below the abutting portion of the fabric holder held at the lower dead point, so that the needle clamp of the needle bar abuts against the abutment portion of the fabric holder with a minimized relative-moving-speed difference between the needle clamp of the needle bar and the abutment portion of the fabric holder; in this way, it is also possible to avoid a sound of collision between the needle clamp and the abutment portion of the fabric holder. Namely, the disclosed sewing machines equipped with the supporting member are constructed to lessen noise sounds generated as the fabric holder moves vertically, by avoiding collision sounds generated during the descending and ascending movement of the fabric holder.
However, the following inconveniences would be encountered by the sewing machines where the fabric holder is vertically moved together with the needle bar via the coil spring provided on the needle bar. First, because the fabric holder is normally urged downward via the coil spring with respect to the needle bar, the holding-down, by the fabric holder, of the sewing workpiece too is effected via the coil spring, and thus, the force with which the sewing workpiece is held down by the fabric holder depends on the intensity of the biasing force of the coil spring. Therefore, the biasing force of the coil spring has to be great enough for the fabric holder to reliably hold down the sewing workpiece, and thus, if the sewing workpiece is leather or other kind of workpiece of a relatively great thickness, the coil spring has to have a very great biasing force. However, if the biasing force of the coil spring is too great, load torque applied to the needle-bar driving shaft (main machine shaft) would increase more than necessary. Particularly, in a case where the sewing machine is, for example, of a multi-head type having a plurality of machine heads, the load torque tends to be very great, which would sometimes adversely influence the sewing.
Second, the biasing force of the coil spring, normally urging the fabric holder downward, would vary in accordance with the up-and-down movement of the needle bar while the fabric holder is located at the lower dead point. FIGS. 13A and 13B are views schematically showing the fabric holder 41 located at its lower dead point in a typical example of the conventionally-know sewing machines equipped with the supporting member. More specifically, FIG. 13A shows a state of the sewing machine immediately after the fabric holder 41 has reached the lower dead point, and FIG. 13B shows a state of the sewing machine when the needle bar is located at its lower dead point. The sewing machine takes the same state as shown in FIG. 13A immediately before the fabric holder 41 is caused to move upward from the lower dead point. Immediately after the fabric holder 41 has reached the lower dead point, the coil spring 43 has not yet been compressed as seen from FIG. 13A, and thus, the biasing force of the spring 43 is relatively weak. However, as the needle bar 3 descends toward its lower dead point, the coil spring 43 is compressed gradually so that the biasing force of the spring 43 increases gradually. The coil spring 43 takes the most-compressed condition and thereby imparts the greatest biasing force when the needle bar 3 has reached its lower dead point. Then, as the needle bar 3 ascends, the coil spring 43 is decompressed so that its biasing force weakens gradually. Namely, the force with which the fabric holder 41 holds down a sewing workpiece varies as the needle bar 3 moves vertically, and thus, the holding, by the fabric holder 41, of the sewing workpiece tends to be unstable.
Further, Japanese Patent Publication No. 2848968 discloses a sewing machine where the needle bar and fabric holder are vertically moved via separate drive mechanisms in order to avoid the above-discussed inconveniences. According to the disclosure of the No. 2848968 patent publication, the fabric holder is provided on the needle bar in such a manner that it is vertically movable relative to the needle bar that is vertically driven in a reciprocative fashion. On a base needle bar provided parallel to the needle bar, there are provided a first elevator member for driving the needle bar and a second elevator member for driving the fabric holder in such a manner that the first and second elevator members are vertically movable. The first and second elevator members are connected to first and second transmission mechanisms, respectively, so that the elevator members are driven to ascend and descend by the respective transmission mechanisms independently of each other interlocked relation to the rotation of the main machine shaft. The first and second elevator members have first and second drive members, respectively, each of which is rotatable about an axis parallel to the base needle bar. The first drive member has an engaging recessed portion engageable with an engagement portion provided on the needle bar, while the second drive member has an engaging recessed portion engageable with an engagement portion provided on the fabric holder. The first and second drive members are each normally held in a predetermined rotational position where the engagement portion of the needle bar or fabric holder engages the recessed portion of the drive member. When the sewing machine is to be brought into a “jump sewing state” where the vertical movement of the needle bar and fabric holder is temporarily stopped, the two drive members are each rotated about the axis to cancel the engagement between its recessed portion and the engagement portion of the needle bar or fabric holder. By thus canceling the engagement, the sewing machine can be brought into the “jump sewing state” where the vertical movement of the elevator members is prevented from being transmitted to the needle bar and fabric holder. Further, the needle bar has a resilient member (or urging member) provided thereon for imparting an upward urging force to each of the needle bar and fabric holder, and, when the sewing machine is in the “jump sewing state”, the abutment portions of the needle bar and fabric holder abut against corresponding stoppers so that the needle bar and fabric holder are held at their respective upper dead points.
In the sewing machine disclosed in the No. 2848968 patent publication, where the fabric holder can be vertically moved, via the second elevator member vertically driven by the rotation of the main machine shaft, without hitting any other member, no collision sound is generated, so that noise sounds resulting from the vertical movement of the fabric holder can be reduced. Further, because the fabric holder is vertically moved with its engagement portion engaging the recessed portion of the second drive member, the vertical movement of the fabric holder is limited (i.e., upper and lower limit positions are defined), so that the fabric holder can be prevented from undesirably lifting up when holding down the sewing workpiece.
The sewing machine disclosed in the No. 2848968 patent publication would present the following inconveniences. Namely, in the disclosed sewing machine, where the needle bar and fabric holder are vertically moved via the separate drive mechanisms, both of the needle bar and fabric holder are “jumped” in order to be temporarily deactivated. However, if both of the needle bar and fabric holder are jumped, collision sounds would be generated when the needle bar and fabric holder abut against their respective stoppers to be held at their upper dead points. Because such collision sounds are generated simultaneously by the needle bar and fabric holder, the collision sounds would become extremely great. Particularly, if the sewing machine is of the multi-head type having a plurality of machine heads, the collision sounds would become much greater in correspondence with the number of the machine heads.